This invention relates generally to digital communications and multimedia information transfer, and, in particular, to scrambling, descrambling, and forward error correction of digital transmission.
Modem data and multimedia transmission systems provide a flexible, cost-effective platform for offering a wide range of telecommunications services to residences and businesses. Some of the envisioned applications provided by these services include Internet access, the ability to communicate with the office while working at home, voice and video telephony, interactive game playing, etc. Whether the transmission medium is coaxial cable, wireless, or optical fiber; and whether the system is multipoint to multipoint, point to multipoint, multipoint to point, or point to point; these systems generally provide for forward error correction (FEC) and data scrambling. FEC is a technique used by a receiver to identify and correct errors incurred in transmission over a communications channel without requiring retransmission of any information by the transmitter. Typically, FEC is implemented by applying an algorithm to data to generate redundant bits at the transmitter, performing the same algorithm on data at the receiving end, and comparing the transmitted calculation with the received calculation. Data scrambling is utilized to ensure that a receiving modem""s clock remains synchronized to the transmitting clock. Scrambling accomplishes this function by ensuring that an adequate number of bit transitions from xe2x80x980xe2x80x99 to xe2x80x981xe2x80x99 and from xe2x80x981xe2x80x99 to xe2x80x980xe2x80x99 occur. A loss of clock synchronization at the receiver relative to the transmitter is known as receiver drift. Receiver drift will normally occur only when the transmitter transmits either xe2x80x9c1""sxe2x80x9d or xe2x80x9c0""sxe2x80x9d without transition for an extended period of time.
Scrambling, although necessary to prevent receiver drift by controlling the one""s density, is also problematic because scrambling increases the error rate. This phenomenon is known as bit error spreading and occurs at the receiver when a descrambler attempts to restore the data to its original, xe2x80x9cpre-scrambledxe2x80x9d state. A bit error introduced into a data stream during transmission is xe2x80x9cspreadxe2x80x9d to other bits because the algorithms utilized to produce the scrambling/descrambling functions rely on each bit, including a bit that is in error, to reconstruct the value of subsequent bits at the receiver and thus restore the original bit values.
The problem of bit error spreading is further complicated with the addition of FEC at the receiver. Conventional systems implementing both scrambling and FEC functions first FEC encode a portion of data to be transmitted. The result of FEC encoding is known as an FEC syndrome. The FEC syndrome is concatenated with the data from which it was derived and then scrambled. Traditionally, FEC was actually performed at the application layer and scrambling/descrambling was performed with the transmission system at the physical layer. A more recent trend is to design systems which perform both FEC and scrambling/descrambling at the physical layer, although preserving the original order of first FEC encoding and then scrambling the data. Performing FEC at the physical layer does not preclude performing additional FEC at the application level.
At the receiver, data is first descrambled and then passed to an FEC decoder. Since descrambling introduces bit errors to bits other than those corrupted during the transmission process, FEC must identify and correct a greater quantity of errors than were introduced during transmission. One consequence of such an arrangement is that system FEC capability may be exceeded, even though errors introduced during transmission could have been corrected by FEC at the receiver were it not for bit error spreading induced by the scrambling/descrambling function. When the error correcting capability of FEC is surpassed, the data is corrupted, cannot be corrected, and therefore requires retransmission. When system designers anticipate bit error spreading, they devote a greater quantity of overhead for FEC. However, this method of compensating for bit error spreading requires a great amount of redundant transmission and is inefficient. What is needed is a method and a system, which includes the scrambling and FEC functions of conventional systems, without adding to system cost or system complexity and still provide for adequate quality of service.
The present invention is a method and apparatus that supports data and multimedia transmission over a flexible, cost-effective platform and enabling a wide range of telecommunications services to residences and businesses. Further, the present invention eliminates the problems associated with bit error spreading and is applicable over a multiplicity of mediums, including coax, wireless, and fiber. Although the present invention is described as being incorporated within a broadband, bi-directional, multiple access, hybrid fiber/coax system, it is not so limited and can equally well be incorporated within a multipoint to multipoint, point to multipoint, multipoint to point, or point to point system.
The present invention is implemented at the transmitter by first scrambling data prior to calculation of an FEC syndrome. After scrambling, the FEC syndrome is calculated and then concatenated with the scrambled data. The concatenated data is transmitted as a subframe to a receiver or receivers. Therefore, the transmitted subframe is partially scrambled (the data portion) and partially unscrambled (the FEC syndrome). Problems with receiver drift due to a loss of synchronization are circumvented by controlling the total length of each subframe and by controlling the length of the FEC syndrome included within each subframe. The present invention inherently allows shorter FEC syndrome length, while preserving the error identification and correction power of traditional systems, due to the avoidance of the deleterious effects of bit error spreading.
At the receiver, data is first FEC decoded and then descrambled. Therefore, FEC is utilized at the receiver to identify and correct bit errors introduced during transmission, prior to a received bit error being descrambled, thus preventing bit error spreading. As a consequence, less overhead is dedicated to FEC for the same degree of error correction capability, and improved FEC performance and system efficiency is achieved without increasing the cost or complexity of the system.